Particulate kaolin products find a variety of uses, including as pigments, fillers, and extenders for use in paint, plastics, polymers, papermaking and paper coating. Kaolin clay, also referred to as China Clay, or hydrous kaolin, is comprised predominantly of the mineral kaolinite, a hydrous aluminum silicate, together with small amounts of a variety of impurities.
Particulate kaolins generally exist in three forms: hydrous kaolin, calcined kaolin and chemically aggregated kaolin. Hydrous kaolin is primarily the mineral kaolinite, which has been mined and beneficiated from natural sources. Calcined kaolins are obtained by processing hydrous kaolin at high temperatures, e.g., greater than 500° C. Chemically aggregated kaolins are particle aggregates having a microstructure resembling that of calcined kaolins produced by treating hydrous kaolin with chemicals. Calcined and chemically aggregated kaolins can show benefits in certain application compositions when compared with hydrous kaolins. However, the benefits associated with calcined and chemically aggregated kaolins are not without disadvantages. The manufacturing costs of calcined and chemically aggregated kaolins are significantly higher than those of hydrous kaolins. The calcined and chemically aggregated kaolins also have the effect of improving certain paper properties while adversely effecting other properties, e.g., strength.
Kaolin has been used as an extender or pigment in paints, plastics and paper coating compositions. Kaolin pigments confer desirable physical and optical properties to such compositions. As flattening (or matting) agents, they help smooth the surfaces of the substrates to which they are applied. As opacifiers, they impart brightness, whiteness, gloss and other desirable optical properties. As extenders, they allow partial replacement of titanium dioxide and other more expensive pigments with minimal loss of whiteness or brightness.
Paper coatings are applied to sheet materials for a number of purposes including, but not limited to, increasing the gloss, smoothness, opacity and/or brightness of the material. Coatings may also be applied to hide surface irregularities or in other ways improve the surface for the acceptance of print. Paper coatings are generally prepared by forming a fluid aqueous suspension of pigment material together with a hydrophilic adhesive and other optional ingredients.
Coatings have been conventionally applied by means of a coating machine including a short dwell time coating head, which is a device in which a captive pond of coating composition under a slightly elevated pressure is held in contact with a moving paper web for a time sufficient to coat the paper before excess coating composition is removed by means of a trailing blade.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.